Heres the scoop,
Picked up a 08 balt turbo about 2 weeks ago. 107k miles, K&N intake, ZZP upper IC pipe, STOCK TUNE (VERIFIED BY P/O)..Compression tested it, 160-165 on all 4, and threw in some OEM AC Delcos per a spark plug thread i read and 100k maintenance.

PROBLEM:
When i test drove the car it was only boosting to about 7 psi and ran like dog poo at idle. Told them i wasn't interested. Got a call the next week and they had the FMIC replaced b/c the mechanic said the old IC had a hole in it. Okay, i see the receipt and old FMIC and i drove it, idled fine, pulled good, saw it went to 15 PSI. The next day bam the CEL came on for under boost, (10 pounds or less). Took it to the shop and they did the wastegate TSB, pulls meh, boosts regularly to 10 psi, not much over 14psi usually, no codes. Today i drove it pretty hard and was pulling back into my neighborhood when i realized my car wont spool more than 5psi.. no CEL but i know it will come on, just didn't have enough time for it to cycle...So this lead me to thinking.. could my stock BPV be the issue here? No leaks in FMIC or pipes/couplers.. and i also notice an audible difference in the BPV, when i first drive it i can hear the "blow off" noise. But once its had a change to warm up, i no longer hear that noise and the car doesn't usually boost over 10 pounds or so. So im thinking maybe its the BPV leaking. Ordered the forge replacement tonight, hopefully that will be the fix. But was wondering if anybody had any other input with these LNFs..

 

5PSI or less is a sort of limp mode known as mechanical boost". You experienced it already but as you said ti didn't throw a code. In order to go to mechanical boost, you have to have less boost than expected for a total of 50 seconds, OR for 4 seconds straight as per AllData.

Any who, you're underboosting and threw the code that pointed them to the wastegate TSB. Did they check the wastegate adjustment, the nipple on the turbocharged boost control solenoid mounted on the turbo, or both? I'd call and ask or check your paperwork.

Also mechanical boost mode goes away on engine restart. Next time that happens, pull over, turn off and back on. Can you go over 5 PSI again? I'd also check your downpipe, a slight clog can reduce boost. Ask me how I know

 

Start with checking your bpv, its the tan looking piece on the intake side of the turbo. iirc it's 3x 10mm and it's spring loaded so carefully back each bolt out working your way around.

When you get the forge unit make sure you use a good high temp synthetic grease and then makesure you lube it every 4-6months to keep it from seizing up.

 

I should try that since my code says something about the bpv.